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David H. Dowell and FriendsSLAC National Accelerator Laboratory
The LCLS Gun
ICFA Beam Dynamics Workshop on Future Light SourcesMarch 1-5, 2010
•The LCLS Injector•Design features of the LCLS Gun•LCLS gun performance•Cathode contamination•Brightest possible beam•Summary and Conclusions
Major Components of the LCLS InjectorDrive Laser
Located in room above gun
S-Band Gun & Solenoid
Dual Feed S-band Linac
+Diagnostics
Design Features of the LCLS Gun & Injector Pulsed heating mitigated with longitudinal coupling and increasing radius of RF aperture
e-beamZ-coupling
Cathode assembly on flange
Dual RF feed and racetrack shape in full cell eliminate dipole and quadrupole RF fields.
•L. Xiao, R.F. Boyce, D.H. Dowell, Z. Li, C. Limborg-Deprey, J. Schmerge, “Dual Feed RF Gun Design for LCLS,” Proceedings of 2005 Particle Accelerator Conference.•C. Limborg et al., “RF Design of the LCLS Gun”, LCLS-TN-05-3, May 2005.•D. H. Dowell et al., “Development of the LCLS RF Gun”, ICFA Dynamics Newsletter, No. 46, August 2008. http://www-bd.fnal.gov/icfabd/Newsletter46.pdf
•Optimum emittance compensation, Ferrario working point•Increased RF mode separation to minimize RF mode beating•Symmetric RF fields in gun and s-band linacs•Z-coupling to minimize pulsed heating for long gun life•Improved cooling for 120 Hz operation at up to 140 MV/m•Full wakefield mitigation in gun-to-linac beamline•Emittance compensation solenoid field with quadrupole correctors•Stable and reliable diode-pumped drive laser•Cathode surface roughness <40 nm peak-to-peak, low dark current
David H. [email protected]
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The Emittance Compensation Solenoid
Relatively strong effect on the beam emittance,especially at high charge
Max on quad corrector
Expected ~setting from mag. meas.
Solenoid requires<0.1% precision
1 nC
Solenoid Int. Field kG-m
0.2%
Norm
alize
d em
ittan
ce (m
icron
s)
Distance from solenoid center (m)
Quad
field
over
prob
e len
gth(g
auss
)
Quad
field
phas
e (de
g)
Gun Solenoid in SLAC Mag. Meas. Lab
bucking coil rotating coil
Quad Correctors:long quads on Gun1long & short on Gun2
long quad wires
short PC quads
Low Charge (20 pC) Measurements and Analysis
Low charge slice emittance meas. at 20 pC
* K-J. Kim, NIM A275(1989)2001-218
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RMS Laser Spot Size (mm)
Nor
mal
ized
Em
ittan
ce (m
icro
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Constant Charge Density Meas.Constant 20 pC Charge Meas.Constant 20pC Charge Fit20 pC Charge Fit + Space Charge Calc.Theoretical Thermal Emittance
Emittance vs. laser sizeat constant charge (20 pC, blue) and
constant charge density (red)
( )2
22222
, sin2)(/9.0
+=+=
− x
xx
A
x
laserrfcathodexscthermaltotaln LI
jE
mcrmsmmm σµσφ
πµσεεε
2, 3mceff
xtheorythermaln
φωσε
−=−
)(/9.0exp, rmsmmmicronsthermaln =−εparameterized
space charge form factor
z
x
z
xx LL
σσµ 0934.00653.0 −=
Space charge emittance*:
=
− z
xx
A
x
laserrfcathodesc LI
jE
mc σµσφ
πε22
2
sin2
j : the current surface densityI A : the Alfven current, 17000 ampsµx : space charge form factorσx : rms transverse beam sizeL z : bunch length (full width)
Summary of LCLS Injector Performance
R. Akre et al., PRST-AB (2007)Y. Ding et al., PRL 102, 254801(2009)
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Em
ittan
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Bunch Charge (nC)
Projected Emittance vs. Charge
Cathode ContaminationThree sources of cathode contamination
•Residual contaminants left by fabrication, handling and storage•Contamination by the gun vacuum
•Ambient vacuum•Operating vacuum
•Contamination during operation due to molecular cracking:• By the laser•By the electron beam
For LCLS contamination by molecular cracking is the most problematic.
Electron beam emission image of the cathode after >1 year of operation. The UV laser beam has left a QE hole at its location.
This will be discussed in more detail in the Cathodes Overview talk on Thursday
The Brightest Beam Possible- How much can the LCLS gun emittance be lowered? -
0 20 40 60 80 100 120 140 160 180 2000
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200 pC100 pC20 pC
Ultimate Emittance Vs. Field & Charge
Cathode Field (MV/m)
Theo
retic
al E
mitt
ance
(mic
rons
/mm
-rms) 20 pC Meas. in LCLS Gun at 50 MV/m
Assume all linear and non-linear space charge effects can be corrected/compensated for,assume the cathode is perfectly flat and the cathode physics is correct. Then the lower limit on the emittance depends on the thermal emittance for the divergence and the space charge limit for the beam size:
( ) ( )rmsmmmicronsEx
thermalcathodeSCLxsmallest −×= /, σ
εσε
23mcW
x
thermal φωσ
ε −=
02
0
density charge surface
επεσ
cathode
bunchcathode R
QE ==
( )2
012 mcEQ
cathode
Wbunchsmallest πε
φωε −=
cathode
bunchSCLx E
Q
0, 4πε
σ =
Summary and Conclusions•Design Features of the LCLS gun and injector:
•Optimum emittance compensation, Ferrario working point•Increased RF mode separation to minimize RF mode beating
•Restores field balance between cells•Reduces correlated energy spread => chromatic aberration in solenoid•Gun RF tuning and field balance is more tolerant to geometry and temperature
•Symmetric RF fields in gun and s-band linacs: elimination of dipole and quadrupole field errors•Emittance compensation solenoid field with quadrupole correctors: Mostly effective at high charge•Z-coupling to minimize pulsed heating for long gun life•Improved cooling for 120 Hz operation•Full wakefield mitigation in gun-to-linac beamline•Stable and reliable diode-pumped drive laser•Cathode surface roughness less than 40 nm peak-to-peak, low dark current, low thermal emittance
•The ultimate emittance based on thermal emission and the space charge limit•Improving the LCLS gun performance:
•“Three issues: cathodes, cathodes, cathodes!!“, G. Neil •QE robustness and uniformity. (see Cathode Overview talk)
•Large (apparent) low charge emittance in gun•High thermal emittance? Poor theory? Poor measurement?•Growth due to surface roughness, aberrations etc.?
•Understand role of solenoid quadrupole in emittance compensation•Correcting for quadrupole space charge fields?•Correcting spherical aberration of solenoid?
•Good diagnostics are essential!And did I mention cathodes?
Emission fromLCLS cathode
Acknowledgements
LCLS Injector Commissioning Team:R. Akre
A. BrachmannJ. CastroY. Ding
D. DowellP. EmmaJ. Frisch
S. GilevichG. Hays
Ph. HeringZ. HuangR. Iverson
C. Limborg-DepreyH. Loos
A. MiahnahriJ. Schmerge
J. TurnerJ. WelchW. White
J. Wu
Special Thanks to the LCLS Gun Group who built the gun and the Commissioning Team who allowed me to show their results.
LCLS Gun Group:Erik Jongewaard & Klystron Dept
Cecile Limborg-DepreyJohn Schmerge
Bob KirbyC. RivettaZenghai LiLiling Xiao
Juwen WangJim Lewandowski
Arnold VlieksValery Dolgashev